1. Field of the Invention
The present invention relates to an image forming material, and, more particularly, to a positive image forming material useful as a positive planographic printing plate precursor suitable for a so-called “direct plate-making” by an infrared laser, by which direct plate-making a plate can be made directly from a digital signal in particular such as from a computer.
2. Description of the Related Art
In recent years, laser technology has rapidly progressed; in particular, higher output and smaller size solid lasers and semiconductor lasers that have an emission region from near infrared to infrared are readily available. These lasers are very useful as an exposure light source when a planographic plate is directly made from digital data such as from computers.
A known positive photosensitive image forming material for use in a direct plate-making by an infrared laser includes a novolac resin or the like as a resin soluble in an aqueous alkali solution. For example, a positive photosensitive image forming material is known which includes a resin that has a phenolic hydroxyl group and is soluble in an aqueous alkali solution such as a novolac resin, a substance that absorbs light to generate heat, and a positive photosensitive compound such as an onium salts, quinone diazide compounds are added (Japanese Patent Application Laid-open (JP-A) No. 7-285275). In the above positive photosensitive image forming material, the positive photosensitive compound, in an image portion, works as a dissolution inhibitor that substantially decreases the solubility of the resin soluble in an aqueous alkali solution. Meanwhile, in a non-image portion, owing to heat, the positive photosensitive compound does not exhibit the dissolution inhibiting effect and the resin soluble in an aqueous alkali solution can be removed by development. In this way, an image is formed.
Furthermore, a positive photosensitive image forming material is disclosed (International Publication WO97/39894 and European Patent Application Laid-open (EP-A) No. 0,823,327). This positive photosensitive image forming material includes a substance that absorbs light to generate heat and a resin whose solubility in an aqueous alkali solution is changed by heat. In this positive photosensitive image forming material, an image portion has low solubility in an aqueous alkali solution, and a solubility of a non-image portion in an aqueous alkali solution is increased by heat. In this way, the non-image portion becomes able to be removed by development to form an image.
In a conventional planographic printing plate precursor, a novolac resin is preferably used since the novolac resin interacts strongly with a dissolution inhibitor, the difference in solubility between the novolac resin in an exposed region and the novolac resin in a non-exposed region is large, and the novolac resin has excellent ink-receiving property. A novolac resin is used also in a positive photosensitive image forming material suitable for infrared laser exposure, for similar reasons. As the novolac resin, in particular, novolac resins obtained by polymerizing phenols such as phenol, cresol, and xylenol with formaldehyde under acidic condition are generally used.
As the dissolution inhibitor, although a variety of compounds are under study, it is known that in particular onium salt type dissolution inhibitors exhibit very strong dissolution inhibiting effect. However, when a general onium salt compound is added, although an improvement in the alkali-resistance in a non-exposed portion can be achieved owing to the strong dissolution inhibiting effect of the onium salt, there exists a problem that the sensitivity is lowered. In order to overcome this problem, a novel photosensitive material that uses a specific onium salt is disclosed. For instance, onium salts disclosed in JP-A No. 2002-278050 and quaternary ammonium salts disclosed in JP-A No. 2003-107688 are known to have excellent characteristics in which high dissolution inhibiting power and high sensitivity are compatible.
However, it has become apparent that there is a problem that as time passes after the exposure, developability of a photosensitive material that uses the above onium-salt type dissolution inhibitor declines and development failure is caused. Such decline in the developability with time lapse after the exposure is problematic in a plate making process and an improvement in the stability of the developability is required. (Hereinafter, the magnitude of developability change after the exposure is expressed by the storability after exposure, and larger decrease in developability is referred to as “worse storability after exposure”.)